Cable Modems (CMs), which can be found in both homes and businesses, communicate to a device which is known as a Cable Modem Termination System (CMTS). The signal between these devices traverses a network composed of both coaxial cable and fiber optic cable, known as a Hybrid Fiber-Coax (HFC) cable plant. The protocol used to communicate between the CMTS and CMs has been standardized by the CableLabs organization and is collectively known as DOCSIS (Data Over Cable Service Interface Specifications). The set of DOCSIS specifications defines the physical layer, media access control layer, and application interface layer.
Radio Frequency (RF) signals sent from the CM to the CMTS are subject to many different types of impairments as they traverse the HFC network. These impairments are typically caused by problems such as loose or corroded connections, unterminated lines, faulty equipment, and other noise caused by sources such as motors and lightning. Most of the impairments are seen in the upstream direction where many CMs can couple noise onto the upstream. In general, the more CMs attached to the same coaxial cable the more noise because the CMs share the uplink and the noise added by each CM link adds to the total noise on the uplink.
As the noisy RF signal traverses the HFC plant, it will be re-amplified by devices such as RF amplifiers and Fiber Nodes, but none of these devices will clean up the quality of the signal.